fix(types): augment types to manage response streams

[JustinBeckwith]

Fixes #2052, and adds a lot more TypeScript 😬

[codecov]

Codecov Report

Merging #2188 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2188   +/-   ##
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  Coverage   84.31%   84.31%           
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  Files           9        9           
  Lines        1594     1594           
  Branches      129      110   -19     
=======================================
  Hits         1344     1344           
  Misses        250      250           

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[alexander-fenster]

[suggestion, feel free to ignore]
should this be imported only if used by the given API? (otherwise, unused imports)

I must admit I have no idea if this applies to APIs in this library, but in GAPICs, we do put all such imports inside if statements in templates.

[JustinBeckwith]

Yeah, in this particular case the import is only being used for the Readable type. It could be required for any of the API calls, if the user uses {responseType: 'stream'}

[sofisl]

I don't think I totally understand the actual changes, but I understand the problem it's solving!